Beyond a single-determinantal description of the density matrix of periodic systems: Experimental versus theoretical Compton profiles of crystalline silicon

Compton profiles of crystalline silicon with a high statistical accuracy and high-resolution (0.11 a.u. in full width at half-maximum) are measured along the three main crystallographic directions and are compared to the predictions of ab initio simulations performed at different levels of theory, within and beyond one-electron approximations. The analysis of the Fourier transform of the Compton profiles reveals the failure of the density matrix extracted from single-determinantal models in reproducing some fine features of the electron momentum density of crystalline materials that can be attributed to the instantaneous Coulomb correlation of the electronic motions. The use of a post-Hartree-Fock periodic scheme allows such features to be satisfactorily reproduced.